An RR turbocharger, for example, has been disclosed by the Brown Boveri reprint Mitteilungen No. 4/5, 1971. For better understanding, this turbocharger is reproduced in FIG. 1. The turbocharger is composed of a radial compressor having a bladed or unbladed diffuser and of a radial turbine. The rotor of the turbocharger consists of a shaft, on which the turbine wheel is mounted at one end and the compressor wheel is mounted at the other end. In the center part of the housing of the turbocharger, the shaft is mounted on pressure-lubricated plain bearings. The housing, for example, may be uncooled or water-cooled. In order to prevent the exhaust gases from penetrating into the bearing space, sealing air is directed to the turbine hub from the compressor side. The path of the sealing air extracted from the compressor is indicated by white arrows in FIG. 1. In addition, this arrangement achieves a reduced heat flow from the turbine wheel through the shaft to the rotor bearing. Due to being supplied with sealing air, the turbine-side bearing arrangement is not subjected to any higher temperatures than the compressor-side bearing arrangement. The temperature level therefore poses no problem even after the engine has been shut down, so that no undesirable coking of the lubricating oil occurs. In order not to obtain any positive pressure in the bearing space, the spaces on both sides of the bearing space are connected to the atmosphere by passages. The bearing housing is effectively protected against the high temperature of the turbine space by an intermediate wall which is cooled by the sealing air. However, this type of sealing-air supply has an adverse effect in particular when a gas is mixed with the compressor air, which may occur, for example, during the compression of biogas. This may then lead to explosive compositions of the sealing air, which ignite in the hot surroundings of the turbine hub. Direct extraction of the sealing air from the compressor is therefore made impossible. The sealing air enriched with the gas should also not pass into the environment. This is problematical, since external air is often not present in the requisite quantity, and separate compressors constitute a considerable price disadvantage. This applies in particular in the case of turbochargers of variable geometry, (VTG), since these turbochargers have an increased sealing-air requirement.